Simulation Of Depletion Mode A-IZO TFT

Compared with the traditional amorphous silicon thin film transistor(a-Si TFT),the depletion mode amorphous indium zinc oxide thin film transistor(a-IZO TFT)has the advantages of high carrier mobility,large area fabrication at low temperature and high transparency.In addition,compared with amorphous indium gallium zinc oxide(a-IGZO),a-IZO lacks the affinity of rare metal Ga element to oxygen element,with higher carrier concentration and higher electron mobility,it has wide application prospects in large-scale,high-resolution flat panel displays and ultra-thin,flexible and transparent electronic devices.However,due to the high carrier concentration in a-IZO thin films,a-IZO TFT usually works in the depletion mode,but most of the devices mentioned in this paper work in the enhancement mode.Few articles discuss the working mechanism of oxide devices in the depletion mode.This paper mainly carries out simulation research on the depletion mode a-IZO TFT,aiming to provide a reference for the study of depletion mode oxide devices.Firstly,a depletion mode single-gate(SG)a-IZO TFT is simulated by Atlas two-dimensional(2D)device simulator of Silvaco.The influence of the thickness of a-IZO layer on the electrical characteristics of the device is discussed in detail.It is found that when the thickness of a-IZO layer reaches 60 nm or more,the turn-off current of the device rises sharply,because the maximum depletion layer width of the device is about 50 nm.In addition,the leakage current increases sharply when the thickness of a-IZO layer reaches 60 nm or more,which is related to the pinning of Fermi level.Because of the existence of deep acceptor defect states,the Fermi level is pinned near the deep acceptor level at the interface,so that the maximum depletion layer width of a-IZO material does not change.Within the maximum depletion layer width,the thickness d of a-IZO layer is 20 nm.The effects of the length of conductive channel and the thickness of gate insulation layer on the electrical properties of SG a-IZO TFT are discussed respectively.Then a density of states(DOS)model is constructed for a-IZO material.Based on the DOS model of a-IZO material,the thickness of a-IZO layer is 20 nm.The effects of several important density of states parameters on the electrical properties of depleted SG a-IZO TFT are analyzed in detail.The simulation results show that the increase of defect number at the end of conduction band makes the turn-on current of SG a-IZO TFT decrease;oxygen vacancy is the main source of free carriers,and its change has a significant impact on the on voltage and sub-threshold swing.With the increase of oxygen vacancy,the negative bias of the device's on voltage and sub-threshold swing become larger,and the gate's control effect on the device becomes weaker.The deep acceptor defect states mainly affect the electrical characteristics of the sub-threshold region.With the increase of the concentration of the deep acceptor defect,the transfer characteristic curve of the device shifts forward.In addition,considering the influence of the interface defect between a-IZO layer and insulation layer,the electrical performance of the device changes greatly.Finally,we optimize the structure of a-IZO TFT,a-IZO TFT with bottom contact structure and a-IZO TFT with double-gate(DG)structure are constructed by use TCAD simulation software.The influence of thickness of a-IZO layer on their electrical characteristics is simulated and compared with SG a-IZO TFT with bottom-gate top contact structure.It is found that the same side of source-drain electrode and conductive channel in SG a-IZO TFT with bottom-gate bottom contact structure,carriers in a-IZO layer has a small contribution to leakage current.Therefore,at the same thickness of a-IZO layer,it show better performance on the on voltage,sub-threshold swing and turn-off current.When the device is turned off,because of the double gate structure,the width of the depletion zone increases and the turn-off current decreases,when the device is turned on,the upper and lower surfaces of a-IZO layer generate conductive channels,and the turn-on voltage is smaller,Compared with SG a-IZO TFT,the double gate structure has stronger current driving ability,about twice that.